Another cam timing qhestion

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Junior Member

A search of the posts brings quite a bit of discussion on cam timing so I hope members are not sick of explanations and someone can clear things up for me.

When discussing cam timing I've always used expressions such as "opening X deg before TDC" or "closing X deg after BDC" however I often see in this forum (and others) things like intake duration 280 - exhaust duration 280 and 110 deg lobe separation. How does this relate to X deg before TDC ?

I know that a duration of 280 deg means the valve is open for 280 deg of the crank shaft rotation and 110 deg lobe separation means that the centre of the intake and exhaust cam lobes are 110 deg apart but this does not tell me when the valve actually starts to open, is cam timing always symetrical or can it also be asymetrical.

When only duration and overlap are given does that mean the timing is symetrical relative to TDC ? if so then drawing up a diagram of a 280 deg duration and 110 deg lobe separation cam would give the following, is this correct ?:

This ties in with my experience rebuilding engines that have no timing marks - I always set the inlet and exhaust rocking at TDC. This was relayed to me many years ago by a guy that reground my Chrysler slant 6 cam he said it was a 35 65 grind not mild but not wild. Drawing up a symetrical cam timing diagram for this gives 280 deg of duration but only 70 deg of lobe separation

Member

Short answer is duration and lobe separation is incomplete info. As you figured out. It completely describes the camshaft in isolation but leave out the all important installation orientation.
In a auto world there are lots of talkers whose knowledge only goes so deep. Everybody speaks jargon, its faster looks like you know what you are doing and avoids looking pedantic or nerdy.

When only duration and overlap are given does that mean the timing is symetrical relative to TDC ?

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It can be but doesnt have to be depending on what the engine is going to do. A tow truck or RV engine will be cammed different then a detroit muscle car.

I have had this info file on my computer for many years. It gives an idea of the effects of changing cam event timing. Doesn't cover the effects of changing duration and overlap. It can really get complicated and i'm of the opinion that if all your model is going to do is run unloaded at the shows, you would have to make huge changes for people to be able to tell. I try to spend more of my energy getting my rings and valves made good. A model will run on just about anything with bumps.

Junior Member

I realise that cam design is complex and will depend on the intended purpose of the engine, for model engines however things are bit different as in most cases model makers (with the exception of those used to power a car or plane) that make working models aren't that interested in torque curves or power output, but that the engine is easy to start and if possible sound like the full sized thing.

This aussie web site https://tighecams.com.au/cars.htm lists the cam timings for many engines and going through it I can see that most cam timings relative to TDC are either symetrical or very close to symetrical. It also has a good page on cam selection and I notice that the cam selection page sorts the cam type/use into duration ranges with no mention made of inlet or exhaust opening/closing, so this would seem to be important perhaps that is why it is referred to so often.

Steve if as you say a model will run with just about anything for a cam then if I ever get around to designing a model engine copying a known working design is the way to go, the aussie web site I mentioned is a good starting place.

I have the comp cams catalog in PDF format. It can also be a good guide. See what you can find in it. I refer to the chevy section because that is the firing order I use so I feel compelled. I would upload it but It is too large a file. PM me and i'll send it to you

Junior Member

Thanks Steve it is much appreciated I'll send a PM.
My son here in Oz has a 67 Mustang, nothing fancy just a plain old 289 2 barrel, it took up space in my garage for the best part of 9 months while we rebuilt the engine and trans. he did most of the work and I only assisted when he needed it - he put in a so called "hot cam" supplied by the guys that refurbed the cylinder heads and we were hard pressed to notice any difference.

Junior Member

I called the guys at Tighe cams to get a bit more info about the advertised specs and spoke to a very helpful guy. He feels that quoting duration and cam lobe separation is not telling the full story so they can keep professional secrets, if they give the full specs then the design is easily duplicated by any one - so they are protecting their designs.

After discussing profiles and cam timing he advised that a calculated low compression ratio of say 5 or 6 to 1 with a reasonable amount of overlap is not a good idea as it will probably make the engine hard to start due to the fact that it will have minimal cranking compression, it is better to run a higher calculated compression ratio to give the engine more cranking compression he suggested 8 or 9 to 1 calculated compresssion ratio with 50deg overlap and inlet open and close at 25 and 65 deg. However given that most start their engines with a cordless drill which probably cranks at 1000rpm or higher this may or may not be relevant for a model engine. He said if running low calculated compression ratio it is better to have minimal or no overlap. Once I finalise my design I'll email him with the specs for some more advise/info

Thats true but your right about it being not relevant for model engines. I run between 5 and 6:1 ratios with 60 degrees overlap and have no trouble starting the engines. With little to no overlap you will change the sound of the engine. The cam is the soul of the engine.